Generally, as an organic electroluminescent element, an element with a structure in which an anode formed of a transparent electrode, a hole-transport layer, an light-emitting layer, an electron-injection layer, and a cathode are stacked in this order on a surface of a transparent substrate is known. In this organic electroluminescent element, when a voltage is applied between the anode and the cathode, light is produced by the light-emitting layer and emitted to an outside via the transparent electrode and the transparent substrate.
The organic electroluminescent element has features such as being self-luminous, exhibiting relatively highly efficient light-emitting characteristics, and being able to emit light of various tones. Thus, utilization as an illuminant for a display apparatus such as a flat-panel display, and as a light source such as a backlight of a liquid crystal display apparatus and for illumination is expected. Some of these elements have been already commercialized. In order for organic electroluminescent elements to be applied more widely to these applications, development of an organic electroluminescent element having superior characteristics of higher efficiency, longer life, and higher luminance has been desired.
Factors that determine the efficiency of organic electroluminescent elements are mainly the three factors of electro-optical conversion efficiency, drive voltage, and light outcoupling efficiency. In terms of electro-optical conversion efficiency, since so-called phosphorescent materials have appeared recently, external quantum efficiency of more than 20% has been reported. This value is, when converted to internal quantum efficiency, considered to be close to 100%. It can thus be said that an example in which a so-called limiting value in view of electro-optical conversion efficiency is considered to have been reached has been experimentally confirmed. In terms of the drive voltage, an element that emits light of relatively high luminance at a voltage that is 10-20% higher than the voltage corresponding to the energy gap has been obtained. In other words, it can be said that there is not much room for improving the efficiency of organic electroluminescent elements by lowering voltage. Thus, there is considered to be not much expectation of improving the efficiency of organic electroluminescent elements based on these two factors.
On the other hand, the light outcoupling efficiency of an organic electroluminescent element is considered as low as 20-30% in general, and there is much room for improvement. Note that this value varies slightly depending on the light-emitting pattern and the internal layer structure. A material that constitutes a portion where light is generated and the surrounding portion thereof has, in general, characteristics such as a high refractive index and light absorbance. Therefore, light cannot propagate effectively to the outside where light-emission is observed because of the total reflection at an interface between layers with different refractive indices, absorption of light by materials, and the like, and as a result, the light outcoupling efficiency is considered to be a low value such as described above. That is to say, light that cannot be used effectively as light-emission makes up 70-80% of the total light-emission amount, and there is a great deal of expectancy for improving the efficiency of organic electroluminescent elements by improvement of the light outcoupling efficiency.
Under such circumstances, a large number of attempts have been made to improve the light outcoupling efficiency. Specifically, many attempts to increase light that reaches a substrate layer from an organic layer have been performed. Since the refractive index of an organic layer is about 1.7 and the refractive index of a glass layer that is normally used as a substrate is about 1.5 in general, the total reflection loss (thin film waveguide mode) that occurs at the interface between the organic layer and the glass layer reaches about 50% of the total emitted light. This value is obtained by point light source approximation and by taking into consideration the fact that light-emission is an accumulation of three dimensionally radiated light from organic molecules. It is possible to greatly improve the light outcoupling efficiency of an organic electroluminescent element by reducing the total reflection loss at the interface between the organic layer and the substrate.
Utilization of interference action is conceivable as one measure to reduce the total reflection loss. Since the thickness of the light-emitting layer of an organic electroluminescent element is relatively thin at several hundred nanometers and is very close to the wavelength of light (wavelength when propagating in medium), thin-film interference occurs inside the organic electroluminescent element. As a result, the intensity of emitted light greatly increases or decreases depending on the thickness of the organic layer because of interference of emitted light inside the element. In order to maximize the emitted light intensity, the light (direct light) that directly propagates toward the light outcoupling side from the light-emitting layer is caused to interfere with the light that propagates toward the light outcoupling side after propagating toward the reflective electrode from the light-emitting layer and being reflected by this electrode so that the light intensity is increased. For example, under the condition that a phase shift n occurs in the light after being reflected by the light reflective electrode from the light before being reflected, the optical film thickness D derived by multiplying thickness d of a film interposed between a light-emitting source in the light-emitting layer and a surface of the reflective electrode by the refractive index n of the film is designed to be approximately the same as an odd multiple of ¼Π of the light wavelength λ. Thus, it is expected that a component amount of the light that is emitted in the frontal direction from the substrate will be maximized. It means that, in this method, light propagating in a specific direction, such as in the frontal direction in which light is easily extracted to the air, for example, is intensified by changing the distribution of light rather than the light being amplified internally. Since an increase in light in one direction is offset by a decrease in light in the other direction, the total light amount cannot be amplified above a theoretical value.
However, in actuality, the phase shift of light is not equal to Π, and light shows more complex behavior. This is because, in an actual organic electroluminescent element, refraction and extinction in the organic layer and the reflection layer exert an influence. This actual phase shift may be expressed as a phase difference θ(λ), for example.
In Patent Document 1, calculating a phase difference θ(λ) using a complex number r(λ) that is calculated from the refractive indices and extinction coefficients of the organic layer and the reflection layer is described. Then, with consideration for this phase difference θ(λ), causing an optical film thickness D from the light-emitting source to the surface of the electrode to satisfy all the following relations so that the component of light that propagates outside the substrate can take a maximum value is described.θ(λ)=Arg(r(λ))2Π/9≦θ(λ)≦15Π/18D(λ)=θ(λ)*λ/4Π0.73D(λ)≦d(λ)≦1.15D(λ)
Note that, in the above equations, λ represents a maximum peak wavelength of emitted light, θ(λ) represents a phase difference caused by the reflective electrode, and d(λ) represents an optical path length between the transmission electrode and the reflective electrode at the wavelength λ.
Furthermore, in Patent Document 2, a microcavity structure is shown as a technique to increase the light intensity by actively utilizing the optical interference effect. The layer structure shown in Patent Document 2 is shown in FIG. 15. In this element, a reflection layer 21, a transparent conductive layer 22, an organic compound layer (e.g., a hole-transport layer 23, a light-emitting layer 24, an electron-transport layer 25, an electron-injection layer 26), a semi-transmissive layer 27, and a transparent electrode 28 are formed on a substrate 20. Then, the thickness of the transparent conductive layer 22 is set at different values depending on the region, and a first region 29a in which the optical path length of the resonator structure is relatively short and a second region 29b in which the optical path length of the resonator structure is relatively long are formed. This structure is effective in increasing chromatic purity and in improving frontal luminance in a single color device.